Exploding Star Caught on Tape

Call it fantastic timing. Early this year, a group of astronomers led by Princeton University's Alicia Soderberg were using NASA's Swift satellite to observe a new supernova-one of those spectacular explosions that mark the end of a massive star's life. This supernova was in a galaxy some 100 million light years away. It was relatively unremarkable, Soderberg admits. But then something extraordinary happened. On January 9, in what some astronomers are calling a remarkable stroke of good luck, another star in their field of view went supernova. "We actually watched the star explode," says Soderberg, who was in Michigan, talking to an audience of fellow scientists about her research when the call about the supernova came from her colleague. This set off a week of scrambling to get astronomers across the globe to point telescopes at the supernova to confirm and better study the phenomenon.

Astronomers have never before seen a star at the first moments of its explosive death. Usually, astronomers miss the earliest flash of a supernova because the explosion is only visible to orbiting x-ray detectors on platforms like Swift. In the 22 May 2008 issue of Nature, Soderberg and her colleagues describe how the supernova's initial burst lasted a few minutes and then faded away. Its power was remarkable. In 10 minutes, the exploding star expelled the about the same amount of energy as the sun puts out in 82,000 years.

"It's incredibly serendipitous," says Harvard astrophysics professor Josh Grindlay, a supernova expert who was not involved in the research. "This almost certainly provides a whole new way of detecting supernovae." Though astronomers have known about supernovas for hundreds of years, the events are rare, only seen about once a century in any given galaxy. They are only visible to the eye or to ordinary telescopes a few weeks after the initial burst, when the supernova begins to shine brightly-sometimes becoming one of the brightest objects in the evening sky.

Supernovae are remarkable events not only for such displays of power but because they culminate a natural process of stellar renewal-sort of like cosmological compost. As famed physicist Hans Bethe said in 1967, upon winning his Nobel Prize, “Stars have a life cycle much like animals. They get born, they grow, they go through a definite internal development, and finally they die, to give back the material of which they are made so that new stars may live.”

What causes a supernova is that the star's core collapses into a tiny, incredibly dense orb. But the rest of the material in the star collapses as well, and when material from the outer layers of the star falls upon this dense core, it bounces off. This forms a shock wave that races out to the star's edge, and breaks out, creating the enormous burst of X rays like the one that Soderberg and her colleagues captured on tape.

The explosion also creates heavy elements and spreads these elements throughout space. The heavy elements in the universe, including those on Earth, originated long ago in supernova explosions. Some of this matter is radioactive, and its decay over time creates the brightly visible display we associate with supernovae. The accidental discovery of the new supernova in January is significant, says Soderberg, because it demonstrates that the first light of exploding stars are these x-ray bursts. They are like early warning beacons heralding the sometimes luminous display that follows.

Bigger and better telescopes proposed for the future will be able to scan the skies and detect these x-ray bursts routinely from all the nearby galaxies. Grindlay, the Harvard astronomer, is the principle investigator on a candidate future NASA mission called EXIST that will scan the entire heavens every few hours and look for nearby black holes and distant gamma ray bursts. If built, the telescope should be able to detect many supernovae in their first explosive moments-perhaps hundreds a year.